The present invention relates to a method for producing a plastic molded semiconductor package, and more particularly relates to improvements in operational reliability and production cost of a plastic molded semiconductor package provided with a heat sink.
In the case of a semiconductor package in which a semiconductor die provided with an integrated circuit is encapsulated in a plastic package, a metallic heat sink is incorporated in the plastic package in order to promote effective dissipation of heat from the semiconductor die. For example, Japanese Patent Opening Hei. 6-5746 discloses a semiconductor package provided with a heat sink partially exposed outside a plastic package. The heat sink supports on its first surface a semiconductor die at a location sufficiently distant from the outer surface of the plastic package, and exposes its second surface opposite to the first surface outside the plastic package. A plurality of electrically conductive leads are mounted onto the first surface in an arrangement juxtaposed along the outer periphery of the heat sink, and these leads are united to the heat sink via an electrically insulating bonding tape. During production of the semiconductor package, the heat sink is bonded to the plurality of leads in a lead frame via the electrically insulating bonding tape.
In the case of the semiconductor package of this prior art, however, use of the electrically insulating bonding tape for uniting of the leads with the heat sink is accompanied with various problems as set out below.
First, relatively high degree of humidity absorption of by bonding agents generally used for the bonding tape tends to allow easy intrusion of outside contaminants into the plastic package, thereby seriously lowering operational reliability of the semiconductor package.
Second, thermal hardening of the bonding agent and dry cleaning of the bonding areas of the leads are additionally necessary in production of the semiconductor package. Generally, the thermal hardening is performed after uniting the heat sink to the lead frame. During this thermal hardening, gas is generated from the bonding agent and adheres as a contaminant to the surfaces of the leads for connection with the bonding wires. The contaminant tends to cause defective connection of the bond wires. In order to avoid such troubles, it is necessary to perform the above-described dry cleaning of the bonding areas of the leads.
Third, connection of the bonding wires tends to cause thermal softening of the bonding agent. Soft bonding agent allows sinking of the leads into the layer of the bonding agent during the process of connection, which causes defective connection of the bonding wires.
Fourth, the electrically insulating bonding tape is relatively expensive and, as a consequence, its use induces rise in production cost.